ILLUMINATION OF OBJECTS; POLARIZED LIGHT 55 



at their zero point. Screen the stage (i.e., the open space between 

 the body tube and stage) and cover the head with a dark cloth. 

 Now observe carefully whether the nicols thus, set are in their 

 position of maximum extinction. This is done by turning one 

 of the prisms the laast amount possible and noting whether the 

 field becomes darker or lighter. Make a number of observations, 

 closing the eyes for a few seconds each time just before looking 

 into the microscope. 



2. Testing the Cross-hairs. - - Having adjusted the polarizer 

 and analyzer to the proper position of crossed nicols as ascer- 

 tained above, attach a low power objective, insert a cross-haired 

 eyepiece and place upon the stage previously centered a prepa- 

 ration of some salt, exhibiting parallel extinction and crystal- 

 lizing in long prisms with straight edges. 1 Center a good crystal 

 and turn the stage until the crystal extinguishes -- i.e., attains a 

 maximum darkness; its edges in this position should be exactly 

 parallel to one of the cross-hairs. Turn the stage through 90 

 degrees; the edge of the crystal must now be exactly parallel 

 with the other cross-hair. If in either case exact parallelism 

 has not been obtained, the cross-hairs of the ocular do not cor- 

 respond to the planes of vibration of the nicol prisms. 



Centering the Stage. - - Before it is possible to make obser- 

 vations relative to the behavior of crystals or other substances 

 toward polarized light or to measure crystal or extinction angles, 

 it is essential that the rotating stage of the microscope be accu- 

 rately centered. 



Place a half slide upon the stage of the microscope, holding 

 it securely in place with a stage spring clip. Focus with a 1 inch 

 or 32 millimeter objective upon the upper surface of the glass 

 slide, moving it about until a tiny defect or mark is found. Move 

 the slide with the fingers until this mark or tiny particle is 

 brought directly under the intersection of the cross-hairs of the 

 eyepiece. Rotate the stage. If the stage is centered the mark 

 or particle will remain under the intersection of the cross-hairs. 

 If not centered, the particle will move in a circle whose circum- 



1 For this purpose allow a drop of a saturated solution of mercuric chloride, or 

 of ammonium sulphate to crystallize very slowly upon an object slide. 



